If you're looking for a small (1.5MB) editing tool to take with you, SunlitGreen Photo Editor is small enough to pack on your flash drive but doesn't skimp on features. You can open and edit multiple files, resize and crop, adjust colours, tweak levels and curves and more. Where SunlitGreen Photo Editor stands apart from most lightweight and portable editors is support for layers and filters. It'd be a far stretch to claim it replaces high-end editors, but for simple correction work made easier by a layer or two, it's an easy to use solution.